This project seeks to develop a high density, minimally invasive electrode array for long-term recording and control of brain activity. Multielectrode arrays are an essential tool in experimental and clinical neuroscience, yet current arrays are severely limited by a mismatch between large or stiff electrodes and the fragile environment of the brain. Chronically implanted electrodes cause ongoing damage to the brain, and an active process of rejection eventually silences neural signals. Failure of chronic implants over long time-scales makes it very challenging to study the neural basis of learning. It also limits the power of brain machine interfaces for human prosthetics or neural stimulation based therapeutics. To minimize electrode damage, the size of implants must be reduced, but multichannel arrays built from the smallest electrodes are impossible to implant due to buckling of the individual fibers as they enter the brain. The proposed recording and stimulating electrode array solves this mechanical problem - achieving a high channel with sub-cellular (5 micron) microfibers distributed in three-dimensional volumes of the brain. To implant the device, individual electrodes are bundled together, strengthening each fiber through mutual support. During implant, the bundle of fibers splays apart and each fiber follows its own separate path into the brain as it is deflected by tissue inhomogeneity. This process preserves the minimally invasive properties of a single fiber. Chronic recordings from prototype designs reveal stable signals, including multiunit recordings with time-scales of months that show minimal drift in neural firing patterns. This project builds on preliminary data to engineer a robust, high channel count (64 channel polyimide) device suitable for both recording and stimulation in basic science studies and eventually for clinical applications. However, due to the minimally invasive nature of this brain interface, the device will be scalable to even higher channel counts. To advance this technology, the project involves a series of aims to optimize the electrode insulator, apply high performance tip coatings, and develop scalable manufacturing processes on a polyimide cable platform. These engineering aims are followed by rigorous benchmarks in vitro and in vivo, including 18 month tests of stimulating electrode capabilities. The project will also demonstrate the potential of the high density, minimally invasive electrode array to trigger diverse activity patterns by shaping the geometry of current flowing through small volumes of the brain.